The US military on Friday defended its decision to drop its most powerful non-nuclear bomb on ISIS positions in Afghanistan, describing it as a “tactical” move.

The GBU-43/B Massive Ordnance Air Blast bomb, or MOAB, was dropped Thursday night on a network of fortified underground tunnels that ISIS had been using to stage attacks on government forces.

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The strike in Nangarhar province near the Pakistan border killed 36 ISIS fighters, Afghan officials said. The US military previously estimated ISIS had 600 to 800 active fighters in the area but was unclear whether it had hoped to strike more.

The blast destroyed three underground tunnels as well as weapons and ammunition, but no civilians were hurt, Afghan and US officials said.